Typically, a vehicle constant speed system detects the speed of the vehicle in real-time, receives speed information previously defined by a user (e.g., 34 miles per hour or more), and controls a throttle body of the engine to thereby allow the vehicle to be driven at a desired speed estimated by the user. In the conventional constant speed system a control operation for maintaining the constant vehicle speed within ±1 mile per hour of a preset speed is as described below.
When the actual vehicle speed increases one mile per hour or more compared to the preset speed, the constant driving speed unit lowers the engine Revolutions Per Minute (RPM) by decreasing the T.P.S. value, thereby, gradually closing the throttle body via a constant driving speed cable. As a result, the actual vehicle speed is restored to within one mile per hour of the preset speed. If the actual vehicle speed decreases one mile per hour or more in relation to the preset speed, the constant driving speed system unit increases the engine RPM by increasing the T.P.S., thereby rapidly opening the throttle body and maintaining the actual vehicle speed within —1 mile per hour of the preset speed.
However, there is a drawback in the conventional constant speed system that has a control system thus described. The engine is often overstrained to maintain the constant speed even when the vehicle rapidly decelerates, such as on an inclined road, thereby causing engine noise and frequent gearshifts. Under a rapid deceleration state on an inclined road, the conventional constant driving speed system carries out an acceleration control to maintain the speed. Thus, the T.P.S. value of the engine changes rapidly and significantly, and the engine noise increases in response to the increment of the vehicle speed. However, when the vehicle speed increases, the actual vehicle speed exceeds the predetermined speed allowance of one mile per hour due to the rapid change of the T.P.S. value. Thus, the constant driving speed system reduces the T.P.S. value of the engine by re-starting the deceleration control and decelerates the actual vehicle speed and the above operation is repeated. In short, when driving on a slope, engine noise continuously occurs due to the presence of a repeated rapid deceleration.